The spread of lodgepole pine (Pinuscontorta, Dougl.) in New Zealand

Forest Ecology and Management 141 (2001) 43±57

The spread of lodgepole pine (Pinus contorta, Dougl.) in New Zealand Nick Ledgard* Scientist, NZ Forest Research Institute, P.O. Box 29237, Fendalton, Christchurch, New Zealand Accepted 02 January 2000

Abstract Lodgepole pine (Pinus contorta, Dougl.) was introduced to New Zealand in about 1880. It is the most vigorous naturally regenerating introduced conifer, which has led to large areas of unwanted spread or `wildings'. Wildings threaten existing indigenous ¯ora and fauna, visual landscape and land use values. The area affected by all conifer natural regeneration is estimated at 150,000 ha of which approximately two thirds is lodgepole pine. Control operations have been undertaken in New Zealand since the 1960s. The high `weed' potential of lodgepole pine, coupled with its low grower and market acceptance in New Zealand, means that the species is seldom planted nowadays. Lodgepole pine spreads more vigorously than other introduced conifers as it cones earlier, is capable of producing seed and wildings at higher altitudes, and has lighter seed allowing dispersal over wide areas. Spread occurs most readily on ungrazed land with low vegetation density, and is least likely to occur in intact, dense vegetation, or where intensive grazing is practiced. Direction and distance of spread from parent trees is predictable. Most wildings occur initially as fringe spread within 50 m downwind of parent trees. Distant spread (measured in kilometers) is usually associated with trees on take-off sites exposed to the wind, such as hilltops and ridges. Spread commonly occurs in `waves', about once every 5 years for fringe spread, and once every 10±20 years for distant spread. Research on seed production, dissemination and the factors in¯uencing seedling establishment has resulted in the development of strategies for the prevention and control of spread. The major aspects to consider for new plantings are siting, planting design and surrounding land management. Where large areas of spread exist, containment is often the most practical recommendation. Eradication is by burning, hand pulling (small seedlings only), felling and application of chemicals to stumps or foliage. Lodgepole pine was the ®rst conifer to attract signi®cant adverse attention due to its propensity for natural regeneration, but concern about the spread of introduced conifers is now general, especially in the hill and high country grasslands of the eastern South Island. This concern has been promoted by new `effects-based' resource management legislation which is currently being implemented by territorial authorities. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Lodgepole pine; Natural regeration; New Zealand

1. Introduction

* Tel.: ‡64-3-364-2949; fax: ‡64-3-364-2812. E-mail address: [email protected] (N. Ledgard).

Forests cover approximately 8 million ha or 29% of New Zealand's land area of 27 million ha (Ministry of Agriculture and Forestry, 1998). Of this area, 1.4 million ha (5%) is in planted production forests of

0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 4 8 8 - 6

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N. Ledgard / Forest Ecology and Management 141 (2001) 43±57

introduced conifers, predominantly consisting of radiata pine (Pinus radiata). This plantation estate is very important to the New Zealand economy. Of the 16 million m3 of roundwood harvested in 1997, over 11 million m3 were exported. Lodgepole pine (P. contorta, Dougl.) is a native coniferous tree of western North America (Critch®eld and Little, 1966), where it is notable for the ecologic extremes covered by its range, and for its `pioneering' ability to invade sites freshly disturbed by extreme storm events or ®re (Agee, 1998). It was introduced to New Zealand around 1880. By 1960 over 10,000 ha of pure or mixed stands (3% of the total area established in exotic forest) contained lodgepole pine (Miller and Ecroyd, 1987). However, because of its slower growth relative to some other introduced plantation species, its susceptibility to wind blow, increasing concerns about unwanted natural regeneration (Hunter and Douglas, 1984; Ledgard, 1988; Forest Research Institute, 1990) and uncertain market acceptance, both the commercial and non-commercial (e.g. for erosion control) planting of lodgepole pine had ceased by 1980. The natural regeneration or `wilding' spread of introduced conifers is now viewed with increasing concern both overseas (Richardson and Higgins, 1998) and in New Zealand, where large areas of the country, particularly in hill and high country, are dominated by relatively treeless, indigenous grassland and shrubland. Wildings spreading from commercial plantations and farm shelterbelts and woodlots are seen to threaten existing landscape character (especially in treeless tussock grasslands Ð Ashdown and Lucas, 1987), native vegetation (by suppressing slower growing plants or vegetation of small stature Ð Harding, 1990; Richardson et al., 1994), grazing potential in traditionally extensively grazed natural grasslands, and catchment water yields (mainly by increasing rates of evapotranspiration Ð Fahey, 1994: Le Maitre et al., 1996). Major recent legislation (the Resource Management Act of 1991), presently being implemented through regional and district government plans, forces prospective tree growers to consider these effects and how they can be avoided, remedied or mitigated (Ledgard et al., 1997). This paper describes the history, ecology and control of lodgepole pine spread in New Zealand. Particular emphasis is paid to the major life cycle stages which in¯uence the rate of invasion (seed production/

dissemination and seedling survival), the management options which have been used to control natural regeneration, and the attitude of the public and administrators towards the spread of wildings. 2. History of lodgepole pine spread Successful natural regeneration (or wilding spread) of conifers was ®rst noted in New Zealand before the turn of the century (Guthrie-Smith, 1953; Smith, 1903 in Hunter and Douglas, 1984). The area affected has increased substantially since the 1950s, probably due to increasing sources and amounts of seed, declining grazing pressure from rabbits (Benecke, 1967) and domestic animals, particularly sheep (Gibson, 1988), and restrictions on burning (Hunter and Douglas, 1984). The area affected by conifer spread is dif®cult to quantify accurately because the density of wildings varies from one to many thousands per hectare. Probably around 150,000 ha have at least 1 wilding/ha. Fig. 1 shows the major areas where introduced conifers are growing. The most commonly planted conifers can be ranked approximately in terms of increasing natural regeneration vigour as follows: muricata (P. muricata) and ponderosa (P. ponderosa) pine
N. Ledgard / Forest Ecology and Management 141 (2001) 43±57

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3. Factors affecting natural regeneration 3.1. Seed production Lodgepole pine is generally considered to be a proli®c seed producer (Lotan and Perry, 1983). In its home range it produces good seed crops as often as 3 out of every 4 years (Stuart et al., 1989). In New Zealand, cones are produced annually, but abundant seed crops occur at irregular intervals (Miller and Ecroyd, 1987).

Fig. 1. Major areas of introduced conifer spread (>100 trees/ha) in New Zealand.

Government declared lodgepole pine a Class B noxious weed in this region (Cooper and Mazey, 1984). Even though most of the dense regeneration has now been cleared, the total area affected has increased and annual control operations continue. To the east of Central Plateau in the Kaweka Range, lodgepole pine occurs over at least another 5000 ha, although, many of these trees were sown as part of revegetation operations on eroded land in the 1960s (Faulkner et al., 1972). In the South Island 40±50,000 ha are affected by conifer spread. In the higher altitude (500±1400 m) natural or semi-natural grasslands (commonly known as high country) Corsican pine is the species with the most extensive spread up to an altitude of 900 m (Ledgard, 1988). In the Canterbury high country lodgepole pine ranks second behind Corsican pine, and ahead of Douglas-®r and European larch (Belton and Ledgard, 1991). Of the total of 17,000 ha affected in Canterbury, lodgepole pine was present in 4350 ha, or 26%.

3.1.1. Age to reproductive maturity In North America, Schmidt and Alexander (1985) reported that some trees produced seed at less than 10 years of age. In Sweden small harvestable seed crops are available after 10±12 years (Andersson and Wennstrom, 1988). In New Zealand, Weston (1957) and Wardrop (1964) reported coning to begin in the subspecies contorta at age 3 (from seed) with viable seed produced from age 5. However, during 20 years of observation, the author has never seen lodgepole pine coning under age 5 (from seed). Wardrop (loc cit) found that 48% of trees produced cones at age 6 and that most trees coned by age 12. Subspecies murrayana and latifolia produce seed slightly later and less proli®cally than subspecies contorta (Weston, 1957). Environment in¯uences age of cone production in conifers. At a low elevation, low rainfall site (50 m, and 700 mm annual rainfall), 10% of lodgepole pine trees produced cones at age 5 and 100% coned by age 8, whereas trees of identical origin and age planted at a higher elevation, higher rainfall site (850 m and 1250 mm) had 1.5% coning at age 6, 26% by age 8 and 80% by age 11 when studies ceased (author, unpublished data). From other similar ®eld observations, Ledgard (1988) proposed that rainfall is the major in¯uence on age of seed production and fecundity. 3.1.2. Number of cones produced In New Zealand cone numbers vary greatly between trees and years. Numbers of 1-year-old cones on a sample of 17, 10-year-old trees planted in uniform sheltered nursery conditions varied from 22±335 with a mean of 128. There was a consistency in quantity of cone production from some trees. Over four consecutive years the least productive tree produced between 0 and 5% of all cones while the most

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3.1.1 above) over ®ve consecutive years contained an average of 49 seeds (range 31±74) per cone (author, unpublished data), of which 33 (67%) were full (range 43±83). This value is similar to Smith et al. (1988) 35 full seeds per cone in high altitude CO, USA, and Gullberg (1980) 30 full seeds per cone in a Swedish study, but more than Hellum and Wang (1985) 20 full seeds per cone in Alberta, Canada.

productive tree produced 14 to 28% of all cones. On a separate single 11-year-old tree which had not shed its serotinous cones, numbers varied annually from 6 to 479 with a mean of 126 (author, unpublished data). Environment not only effects age of ®rst cone production (see Section 3.1.1), but also numbers of cones set. Eleven-year-old trees at the low elevation, low rainfall site averaged 70 cones per tree (range 10± 253) while trees of the same origin and age and similar height (mean difference 0.2 m) at the higher site carried only 18 cones per tree (range 0±120). Lodgepole pine can produce seed at higher altitudes than most other conifers (Lotan and Perry, 1983). At the Craigieburn Range in Canterbury (Fig. 1) the species regenerates above the natural Nothofagus timberline at 1370 m (Ledgard and Baker, 1988). On a slope where it was planted alongside Douglas®r and Corsican pine, lodgepole pine produced seed and seedlings at 1200 m, whereas the upper limit of Douglas-®r coning was 1000 m and no wildings were seen above 1050 m. No Corsican pine cones were seen above 800 m. In New Zealand the only other introduced conifer known to seed as high as lodgepole pine is dwarf mountain pine (Pinus mugo).

3.1.4. Seed quantity per tree Table 1 gives average (per species) and maximum (individual tree) estimated full seed production in 1 year for lodgepole pine compared to six other conifer species growing adjacent to each other at the Craigieburn Range, New Zealand (author, unpublished data). Variable tree age in Table 1 makes comparison dif®cult, but lodgepole pine appears to be a species most capable of proli®c seed production at a young age. 3.1.5. Seed weight Lodgepole pine seed is smaller than that of any other pine except P. banksiana (Lotan and Perry, 1983). In New Zealand and North America the number of seed/kg ranges from 200,000 to 300,000 (Lotan and Perry, 1983; Miller and Ecroyd, 1987). Seed of subspecies latifolia and murrayana are heavier than those of subspecies contorta.

3.1.3. Seed viability and number of full seeds per cone Miller and Ecroyd (1987) record seed viability for lodgepole pine in New Zealand to be generally between 70 and 90%. Multi-cone samples taken from individual trees (aged 7±11) at Rangiora nursery (the low elevation, low rainfall site mentioned in Section

3.2. Seed dissemination The inland subspecies latifolia and murrayana and the coastal subspecies bolanderi produce mainly

Table 1 Estimated numbers of viable (filled) seed per tree produced in 1 year for seven introduced conifer species in New Zealand (author, unpublished data) Species

Lodgepole pine (1250 mm annual Rainfall) (Pinus contorta) Lodgepole pine (700 mm annual Rainfall) (P. contorta) Corsican pine (P. nigra) Scots pine (P. sylvestris) Radiata pine (P. radiata) Mountain pine (P. uncinata) Dwarf mountain pine (P. mugo) Douglas-fir (Pseudotsuga menziesii)

Tree age (years)

10 10 21 15 11 20 18 16

Estimated Mean numbers of filled seed per tree (n)

Maximum number of filled seed per tree

395 6630 22300 3415 914 1179 3769 421

1566 17353 44856 12341 3086 2467 10461 2792

(52) (17) (10) (10) (14) (10) (10) (10)

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serotinous cones which will not open readily on the tree until either the tree is felled and/or the cones are strongly heated Ð usually by ®re (US Forest Service, 1965). In New Zealand, wild®res are not common, so seed from trees with serotinous cones is often not released until cones open following tree felling. Serotinous cones can persist unopened on trees for many years, and on felled trees (particularly in dry environments), seed may remain viable in cones far longer than free seed (US Forest Service, 1965; Keeley and Zedler, 1998). Hagner (1990) reported little decline in viability, while Lotan and Perry (1983) reported increasing loss of viability with cone age. Although no studies have been undertaken in New Zealand, cone serotiny probably contributes to the slower spreading nature of subspecies latifolia, murrayana and bolanderi by reducing the annual availability of seed. Non-serotinous cones open shortly after they ripen. In the Northern Hemisphere this is usually in the early autumn (late September), but seed release may extend over several months (Lotan and Perry, 1983). In New Zealand's high country, most seed is also released in early autumn (March and April) (author, unpublished). Some (<5%) remained in the base of the cones until the following spring (November). G. Rogers (personal communication) using seed traps in New Zealand's Central Plateau region, recorded a major release of seed in May. In common with the majority of other pines, lodgepole pine seed is winged and dispersal is mainly by wind (Lanner, 1998). In North America, Schmidt and Alexander (1985) recorded that most seed falls within 60 m of the parent stand. This appears to be the case in New Zealand, although the light weight of lodgepole pine seed means that wind can also carry seed for considerable distances. Strong wind gusts are a feature of the climate in those areas where lodgepole pine has been most frequently planted (McCracken, 1980). These winds are often warm and provide the drying conditions necessary for cone opening. Consequently there are many records of wildings occuring many kilometers from the nearest seed source (Wardrop, 1964; Cooper and Mazey, 1964; Hunter and Douglas, 1984). There are records of spread out to 40 km (author, unpublished; W. Lee, personal communication). Observations indicate that, if left undisturbed,

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such outlier trees will produce viable seed and subsequent wildings by age 15. 3.3. Seed predation In the Northern Hemisphere seed predation by squirrels, birds, mice and insects can be signi®cant (Lotan and Perry, 1983; Keeley and Zedler, 1998). In New Zealand, there are no records of predation of lodgepole pine seed in cones, but seed losses can be major after dissemination. Hayward (personal communication) placed groups of 100 seeds in 29 sparsely vegetated montane sites and used diagnostic features of seed remains to estimate predation by birds and mice. Twenty-two sites were predated, 54% by mice (maximum seed loss 100%) and 15% by birds (maximum 50%). Of the ®ve species used in these trials, lodgepole pine was the least affected, possibly due to it having the smallest seed. On mineral soils Ledgard (1979) reported seed losses of 15% to insects and lesser losses to mice. 3.4. Establishment Although viable seed can be produced at age 5, few wildings appear under trees 8±10 years old (Wardrop, 1964). Bastow-Wilson (personal communication) observed that although trees on Mid Dome in Otago (Fig. 1) seeded regularly at age 7, little `effective' regeneration occurred before age 15. Ledgard (1988) noted similar delay in Douglas-®r. The delay could be due either to low numbers of seed being produced or to delayed germination, although Langer (personal communication) found that over 80% of viable seed germinated in the ®rst season on bare mineral soil at Rangiora nursery and on lightly vegetated, moist sites (1000±1300 mm annual rainfall) in the Craigieburn area (Fig. 1) with no germination occuring after year 4 (14%). On lightly vegetated, drier sites (700± 800 mm) the bulk of germination was delayed to years 2 and 3 (56 and 39%, respectively), but none occurred in year 4 or later. Faulkner et al. (1972), working in moister North Island conditions, reported that most seed germinated within 3 years. 3.4.1. Frost heave Schmidt and Alexander (1985) reported that seedlings establish best in mineral soil. However, in

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montane New Zealand environments, where most spread occurs, survival of conifer seedlings on bare mineral soils is usually very low due to frost heave over winter. No data are available for lodgepole pine, but in the Craigieburn Range, Ledgard (1979) reported that for P. mugo, 44% of ®rst year seedling losses were due to frost heave. 3.4.2. Seedling predation In seeding trials (not involving lodgepole pine) Davis (1996) reported that 50±100% of losses of pine seedlings were due to rabbits (Oryctolagus cuniculus) in the ®rst year after sowing seed in depleted, short tussock grasslands. Belton and Ledgard (1991) found that high populations of rabbits will prevent seedling establishment, and Crozier and Ledgard (1990) noted that they will browse lodgepole pine in preference to other conifers. Lodgepole pine is more palatable to sheep than most other conifers in New Zealand (Crozier and Ledgard, 1990). Benecke (1967) found that a stocking rate of one sheep/ha was suf®cient to prevent establishment of lodgepole wildings. However, since most lightly grazed grasslands are not grazed uniformly (Harris and O'Connor, 1980), wildings can establish on sites less favoured by sheep (Ledgard, 1988). Crozier and Ledgard (1990) found that wildings were only killed by grazing if the stem was completely severed at or below the needle collar (base of live needles) before the seedlings were 2 years old. Older seedlings were harder to graze to needle-collar level and usually recovered after grazing. Sheep graze more intensively and closer to the ground than cattle and are therefore more effective in controlling wildings (Gibson, 1988). The brush-tailed possum (Trichosaurus vulpecula) will eat the 2-year-old bark of mature trees causing dieback of stem and branch tips and possibly affecting cone production. Subspecies bolanderi is more susceptible to possum browse than subspecies contorta (Wishart, 1984). 3.4.3. Competition New Zealand observations have shown that conifer establishment from seed is highest on mineral soil (Langer, personal communication) where there is no winter frost heave (Ledgard, 1979), and on lightly vegetated sites. Establishment success declines with increasing vegetation cover. Watt (1986) found fre-

quent, aggressive and fast growing seedlings at sites with a sparse, short vegetation cover. Numbers were reduced in taller shrubland, and none were found under closed forest canopies. Allen and Lee (1990) observed that seedlings established best where scattered tall tussock grasses sheltered sites partially vegetated with grasses and ¯atweeds. Seedling numbers declined rapidly as density of tussock and surrounding grassland vegetation increased. Benecke (1967) followed the survival of 1-year-old seedlings in dense, improved (fertilised) grassland and in partially-open unimproved grassland. After 18 months no seedlings survived in improved grassland, whereas 94% survived in the unimproved grassland. Davis (1989) direct drilled lodgepole pine seed into improved (sown and fertilised) and unimproved grassland. Of the viable seed, 47% germinated to young seedling stage. In improved grassland none survived to the end of the ®rst season except where a herbicide treatment was added (75% survival). Seedling survival in unimproved grassland was 100%. Langer (personal communication) followed seedling survival under an intact forest canopy, in canopy gap light wells, in open shrubland (1 m tall) and in adjacent intact grasslands. After 6 years seedling numbers were highest (43% of seed sown) in the open shrubland, followed by the grassland (6.3%), the canopy gap (0.3%) and the intact forest (0%). The better establishment in open shrubland was probably due to there being fewer competitive grasses and increased shelter from wind and excessive summer heat. A notable feature in both the shrub and grassland sites was the very low mortality after the ®rst year. Under the canopy gap, seedlings never prospered, looked chlorotic and after 6 years averaged only 5±7 cm in height, compared to 128 cm in the shrubland and 55 cm in the grassland. Symbiotic, mycorrhizal fungi are known to assist in conifer seedling establishment in the New Zealand high country (Davis, 1996). Their role in lodgepole pine establishment and growth is currently being researched in New Zealand and offshore (Read, 1998). Lodgepole pine is capable of regenerating freely after felling of individual trees and plantations (Slow, 1954), probably due to the release of large numbers of seed onto disturbed soil. On the other hand, grass invasion after felling can be so rapid that no woody plant regeneration is possible for many years (Ledgard and Baker, 1988).

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4. Pattern and timing of spread The most important factors in¯uencing the pattern and timing of spread are annual variation in seed production, climatic variations affecting seed dissemination (temperature and wind strength) and seedling establishment (frost and drought), the nature of the ground surface and density of vegetation competing with young seedlings, location of seed source (relative to the prevailing winds) and surrounding land management relative to domestic (mainly sheep) or wild animal (usually rabbits) grazing pressure. Natural regeneration is most likely to occur on ungrazed land with low vegetation density downwind of a seed source; it is least likely to occur in well established dense vegetation or where intensive grazing is practised. In New Zealand, sheep are the most important grazing animals. Most seed dispersal is by wind in the autumn. The direction of spread is largely predictable, because of the prevalence of westerly winds in New Zealand. Seed dispersal and therefore wilding spread is usually to the east of the seed source. On relatively ¯at country most spread occurs as fringe spread within 50 m downwind of the source. Distant spread beyond a few hundred meters originates most frequently from trees on ridges, hilltops, and sites on or adjacent to north or west-facing slopes. These are often called take-off sites (Fig. 2). Distant spread usually gives rise to scattered, lone outlier trees (Ledgard, 1988). Cooper and Mazey (1984) noted 5-year `waves' of fringe spread adjacent to parent trees in the North Island's Central Plateau area. They attributed this to heavy seed production from mature trees occuring every 4±5 years. In the drier areas of the South Island the interval between waves appears longer (author, unpublished data), with successful establishment by distant spread occuring perhaps only once every 10± 20 years (Fig. 2). Characteristically therefore, spread in New Zealand occurs in a step-wise pattern (Ledgard, 1988; Ledgard, 1993), similar to that recorded for other pines by Richardson et al. (1994) and Richardson and Higgins (1998). 5. The control and management of spread In New Zealand, lodgepole pine production and soil protection plantings and sowings are now the sources

Fig. 2. Typical sequence of lodgepole pine spread in agriculturally undeveloped tussock grasslands in New Zealand (adapted from Ledgard, 1988 and 1993).

for considerable areas of unwanted spread. Despite large areas of unwanted regeneration being removed by local government bodies and volunteer conservation groups, the potential for further spread remains high. Even if the species was more desirable from a commercial perspective, its propensity to spread would probably restrict its use. The 1991 Resource Management Act requires land managers to consider the possible ill-effects of any land use, and local government bodies implementing the Act have indicated that the risk of wilding spread must be minimised before they will grant permission for tree establishment in sensitive areas (Ledgard, 1997). The emphasis of the new legislation is aimed at prevention of spread from new plantations rather than control or removal of existing spread. Prevention is greatly assisted by the relative predictability of the timing and sequence of regeneration of introduced conifers, which gives land managers opportunities to intervene at strategic times to minimise the risks of unwanted spread. Guidelines for the control or management of wilding spread have been produced to

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Table 2 Questionnaire used calculating spread risk from new plantings (adapted from Ledgard, 1994)a,b Species Ð spreading vigour Radiata and muricata pine Ponderosa pine and larch Corsican pine and Douglas-fir Scots pine and Lodgepole pine (P contorta) Enter socre (1, 2, 3 or 4) here

1 2 3 4 &

Species Ð palatability Radiata and ponderosa pine Lodgepole pine and larch Scots pine and Douglas-fir Corsican pine Enter score (1, 2, 3 or 4) here

1 2 3 4 &

Siting Flat (<108)/sheltered, or slopes facing NE to SSW (compass 458 to 2008) Flat (<108), partially exposed to N and W (2008 to 458) Flat (<108), fully exposed to N and W (2008 to 458) Take off site i.e. ridgetops, on or at base of slopes (>108) or undulating land fully exposed to N and W (2008 to 458) Enter score (1, 2, 3 or 4) here

1 2 3 4 &

Downwind landuse Ð within 200 m Developed pasture/regular mob stocking (sheep) or closed canopy scrub/forest Semi improved grazing/occassional mob stocking Extensive grazing only No grazing Enter score (1, 2, 3 or 4) here

1 2 3 4 &

Downwind landuse Ð from 200±400 m (if 1 or 2 scored in `Siting'), or, from 200 m±2 km (if 3 or 4 scored in `Siting') Developed pasture/regular mob stocking (sheep) or closed canopy scrub/forest Semi improved grazing/occassional mob stocking Extensive grazing only No grazing Enter score (1, 2, 3 or 4) here

1 2 3 4 &

Total score

&

a

A score of 12 or more indicates a high spread risk. A high risk does not necessarily mean `No trees'. A change of species, or siting, or downwind land management can significantly lower spread risk. Or a commitment to wilding removal can be made Ð this is not onerous, particularly with regard to long distance spread from plantings on flat land (Q. 3-scores 1, 2 or 3). b Long distance spread: this is likely if a score of 3 or 4 in `Siting' (Q. 3) is followed by a 3 or 4 in `Downwind landuse' (Q. 5), especially if larch, Douglas-fir, or Corsican, Lodgepole or Scots pines are involved.

assist councils and forest managers (Ledgard and Crozier, 1991). 5.1. Calculating likelihood of wilding spread A simple form (Table 2) has been developed for calculating wilding spread risk from new plantings (Ledgard, 1994). The form seeks answers to ®ve questions concerning species, their spreading vigour and palatability to browsing animals, siting relative to exposure and likelihood of seed blow, and surrounding land use relative to vegetation cover and grazing

intensity by sheep. Scores of 1 (low risk) to 4 (high risk) are invited for each question. The highest possible total score is 20 and a score of 12 or more indicates a high spread risk. Options involving changing the species, the planting location or the surrounding land management can be tested using the assessment sheet. 5.2. Reduction and prevention of spread Tree spread may be reduced or prevented by one or more of the following:

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5.2.1. Removal of existing and potential sources of spread Although this is the most obvious preventative measure, it is often too expensive. In New Zealand the priority is moving towards preventing spread in new areas, with particular focus on new plantings and lone outlier trees. Methods for removal are described in Section 5.5.

5.2.6. Biological control Little research has been carried out in this ®eld (cf., South Africa Ð Richardson, 1996). However, Brockerhoff and Kay (1998) have identi®ed three insects which they believe deserve testing for host-speci®c control of lodgepole pine.

5.2.2. Choice of species and sites The Lodgepole pine subspecies bolanderi, murrayana and latifolia spread less readily than the subspecies contorta (Hayward and Wishart, 1975). Tree planters are encouraged not to plant spread-prone species such as lodgepole pine, on or adjacent to, take-off sites upwind of unmanaged areas, unless the risk of spread is acceptable to all those likely to be affected (Ledgard and Crozier, 1991; Ledgard, 1997; Ledgard et al., 1997).

Because of the large size and inaccessibility of many wilding affected areas, and the cost of eradication, containment is often the most practical option. It usually involves a central containment area within a carefully managed surround (Ledgard, 1988). Spread risk can be lessened further by removing source trees from take-off sites within the containment area. As an added precaution, particularly on ¯at sites where most fringe spread is likely to originate from seed from edge trees, boundaries may be planted with two or three rows of less spread-prone species such as radiata and ponderosa pine. Within the surrounding control zone no trees are allowed to reach the age of seed production. Hand pulling at an early age is recommended, being a simple, relatively inexpensive operation. This should be carried out every 3±5 years, or missed seedlings will become too large for hand pulling and other more expensive techniques will have to be employed. Such regular operations require considerable long-term management commitment.

5.2.3. Planting design Ledgard and Crozier (1991) suggested that seed is more readily produced and disseminated from edge trees, which have deeper green crowns and are closer to potential regeneration sites. If this is the case, it should be possible to lessen spread risk by planting species that are less prone to spread (such as radiata and ponderosa pine) around stand margins. 5.2.4. Promoting growth of surrounding vegetation The opportunities for spread can be limited by seeding and fertilising around a forest margin. Seeding (particularly of fast growing grasses and legumes) in combination with fertiliser will increase vegetative cover and markedly limit the sites available for wilding invasion (Benecke, 1967). 5.2.5. Grazing Extensive but regular `range' grazing with sheep will signi®cantly limit (but not eliminate) spread, often to the extent that other control requirements are minimal. Exclosure plots show that, even at low numbers (<1/ha), they have an important role in reducing wilding success, but periodic intensive grazing (mob stocking) before seedlings reach 2 years of age is necessary to prevent all new establishment (Crozier and Ledgard, 1990). Cattle grazing is not as effective as sheep grazing (Gibson, 1988).

5.3. Containment

5.4. Eradication Different techniques, from grazing, burning, hand pulling and felling, to killing with the aid of chemicals, can be used for removing wildings, and are appropriate for different terrains and tree ages and densities. 5.4.1. Grazing Wildings are dif®cult to kill by grazing after they are 2 years of age; older trees will only be severely hedged by grazing. Crozier and Ledgard (1990) showed that simulated grazing was more effective in autumn than in summer. 5.4.2. Burning Because of variable terrain, fuel density and weather, control by ®re is often not complete. A poor

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burn can promote a new crop of wildings (Hunter and Douglas, 1984). It will not destroy all fallen seeds and will encourage the opening of serotinous cones. Best results have been achieved when the fuel was very dense or dry. Burns of lodgepole pine have been successful following crushing (Hogg and Garrett, 1975 in Hunter and Douglas, 1984) and chemical spraying (see Section 5.4.6). However, the impact on other environmental values must be considered. If successful, ®re is the cheapest removal technique. Burning is a very important element of wilding tree control in South Africa (Richardson and Higgins, 1998). 5.4.3. Hand pulling This is recommended for seedlings smaller than 0.5 m tall. 5.4.4. Felling Felling is only effective if all live foliage is removed from any remaining basal stumps. The most common tools are slashers, pruners and axes for small trees, and motorised scrub cutters and chainsaws for larger trees. When using chainsaws in stony terrain, small hand axes are often carried to remove live branches growing close to ground level. Amongst felled trees, dense wilding regeneration can occur as a result of surface disturbance exposing mineral soil and increased seed fall as cones in the slash open. However, the density of regeneration can also be reduced by thick debris (slash) and competition from invading herbaceous vegetation. 5.4.5. Felling and application of chemicals to stumps If all live foliage cannot be removed physically due to stones or thick vegetation around the stump, it is often more ef®cient to cut the trees higher up the stem and then kill the stump with chemicals. Four chemicals have been tested on lodgepole pine stumps (Crozier et al., 1988). Sodium chlorate (applied as a powder), ammonium sulphamate, 2,4-D and glyphosate (applied in liquid form) were all effective. Sodium chlorate was the fastest and most easily handled in the ®eld, but it is a powerful oxidising agent and is classi®ed as a hazardous substance. 5.4.6. Herbicide treatment Preest (1985) used mixtures incorporating 2,4-D/ glyphosate/paraquat and diesel, and bromacil granules

to kill scattered lodgepole pine seedlings up to 3 m tall. A double application of bromacil (in the form of a wettable powder) was used successfully to kill dense lodgepole pine stands up to 18 years old and 10 m tall prior to burning (Rodgers, personal communication). Davenhill and Preest (1974) found bromacil and karbutilate to be the most effective of seven chemicals in killing young lodgepole pine trees when applied to the root zone during the growing season. These chemicals also killed off indigenous vegetation in the affected areas. In a test of ®ve herbicides, Crozier (1990) killed 1-year-old seedlings with glyphosate and picloram and found summer application to be more effective than winter treatment. Metsulfuron, triclopyr and 2,4D were much less effective. In recent years metsulphuron (at 100±120 g/ha) in mixture with a surfactant and either glyphosate (at 5 kg/ha), or paraquat (at 5.6 l/ha) has been used to broadcast spray and kill wildings under 3 m tall. 5.5. Long-term commitment Long-term commitment is a prerequisite for successful wilding control. All wilding control operations require at least two removal `sweeps' separated by some years (usually 5±10) to achieve success. The ®rst sweep aims to remove all visible wildings, but inevitably small seedlings may be overlooked and there may be some delayed germination of seed in the soil. The temptation has been to organise follow-up removal operations too soon after the ®rst. Faulkner et al. (1972) found most lodgepole pine seed to have germinated within 3 years of sowing, whilst Langer (personal communication) found seed germination in the ®eld to be completed within 4 years. Therefore, all remaining seedlings should be visible within 7 years of the ®rst sweep, by which time very few are likely to have seeded successfully. The second, and possibly ®nal, removal sweep could be left to this time, unless there is evidence that seedlings are coning. 5.6. Costs of wilding removal The most signi®cant in¯uences on wilding removal costs are wilding density, tree size, and the accessibility and surface type (rock or soil) of the terrain to be covered. Consequently, costs vary greatly from site to site. New Zealand labour costs in the type of remote

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country where wildings usually occur are at least US$ 12/h. Operational experience from a range of sources has found the following:  Approximately 20 man h/ha have been needed to clearfell dense wildings by chainsaw, and 5 h/ha for widely scattered small wildings removable by handpull or hand tools.  Individual tree cost was around US$ 0.65 per tree to clear wildings at a density of 33/ha and ranging from 0.2 to 5 m in height, using a combination of chainsaws (for trees >1 m tall) and a metsulphuron/ paraquat foliage spray for smaller trees.  Recent costs for aerial broadcast spraying natural regeneration prior to commercial planting in the North Island was US$ 150/ha using a metsulphuron/ glyphosate/surfactant mix.  For wildings on country with difficult foot access, a helicopter `skid-hop' technique can be used. This involves ferrying an experienced chainsaw operator from tree to tree. A small (Robinson) helicopter plus chainsaw operator costs around US$ 250/h, and where wildings are less than 1 for every 5±10 ha, 1 km2 can be covered for US$ 100.

6. Attitude of the public and administrators Widespread anxiety about the spread of wilding conifers is relatively recent, compared to longer standing concerns about the impact of introduced plantation conifers on the visual landscape and traditional pastoral uses. Therefore, there is a wide-spread lack of knowledge about the topic, and a lot of misconceptions, both amongst the public and at land administration and management levels. 6.1. Public attitude The public attitude to wilding presence is dif®cult to separate from their attitude to introduced conifers generally. Over much of New Zealand, plantations of conifers (and their silviculture) are accepted as normal land use. However, in traditionally pasturedominated farm lands there is more resistance, particularly concerning the drier eastern hill and high country (Swaf®eld, 1994), where extensive pastoral-

53

ism on virtually tree-less terrain has been practiced ever since European colonisation (approximately 150 years). Declining returns from pastoral products and increasing costs are making traditional sheep and cattle farming less viable, and landowners are increasingly interested in alternative land uses such as forestry (Ledgard and Belton, 1985; Belton, 1993). Whilst many of those dependent on the land show greater interest in forestry, there is growing public concern about the possible adverse impacts of conifers, which include the spread of wildings (Harding, 1990). Organisations which have spoken out about the adverse impacts of wilding spread on visual landscape and existing conservation values include the Department of Conservation, Royal Forest and Bird Protection Society, Regional Conservation Boards and the Federated Mountain Clubs of New Zealand. 6.2. Attitude of administrators Although the `weed' potential of lodgepole pine has been of®cially recognised since the 1960s (Cooper and Mazey, 1984), and legal moves to restrict its use were initiated in 1983 when the Government declared the species a Class B noxious weed in the central North Island, more widespread recognition of the adverse effects of wilding spread by territorial administrative bodies is comparatively recent. The introduction of the Resource Management Act (RMA) in 1991 reoriented land use and planning considerations in New Zealand from mainly promoting the physical, economic and social well-being of communities (Perkins and Memon, 1993), to a more focused concern for enabling the sustainable management of natural and physical resources. The `Purpose' of the Act (Section 4, RMA, 1991) includes a major requirement to avoid, remedy or mitigate any adverse effects of land use on the environment. Responsibility for the implementation of the RMA is largely devolved to regional and district councils. Their regional and district plans are the local expression of the RMA, and aim to promote sustainable land use at those levels. In the hill and high country of eastern parts of New Zealand, many district plans have classed plantation forestry as a controlled or discretionary activity, which means that a Resource Consent from the council is likely to be required before any planting can take place. A Resource Consent application must

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include an assessment of any actual or potential effects of forest planting on the environment, and ways in which any adverse effect may be avoided, remedied or mitigated. Applications can be expensive (Ledgard, 1997). Many district plans require prospective tree planters to control or remove wilding spread outside new plantations. For example, the draft Mackenzie District plan (1997) states that ``It shall be the responsibility of forest owners, occupiers, lessees and licensees or other persons responsible for the forestry to eliminate tree spread and growth of wilding trees emanating from that forest on all land within 500 m of the forest edge''. Other plans have tried to include more de®nitive conditions involving the non-use of spread-prone species (such as lodgepole pine) on certain sites, and appropriate planting design and surrounding land use to minimise the risk of unwanted spread. 7. Education and awareness There is common agreement that more education and awareness about wilding conifer spread is urgently needed, and that if the facts were better known, most land owners and managers would be prepared to plan for, and manage trees, so that the risks of unwanted conifer spread are avoided or minimised. 8. New Zealand .v. off-shore experience Richardson et al. (1994) and Richardson and Higgins (1998) review the status of pines as alien invaders in the southern hemisphere and explore the various factors in¯uencing their spread and its control. Lodgepole pine has the attributes of small seed with low seed-wing loadings, a short juvenile period, and short intervals between large seed crops, which these authors use to characterise an invasive pine species. Another characteristic, ®re resistance, is of much less importance in New Zealand than in South Africa or Australia. The higher and more even annually distributed rainfall means fewer ®res, and they are not encouraged for risk of increasing erosion potential. Therefore burning is not a frequently used wilding control tool for land managers in New Zealand (cf., South Africa).

Apart from the species attributes discussed above, the same authors list the major site factors which determine whether an introduced pine will spread from planting sites, as being residence time, extent of planting, distance from equator, ground-cover characteristics, disturbance and resident biota. In New Zealand, the most important are the last three Ð commonly encompassed by the term `surrounding land management'. This term implies, as Bond and Richardson (1990) point out, that in modern times, changes which alter the suseceptibility of any site to invasion are often human induced. The manipulation of factors such as grazing pressure and vegetation density are very important components of control strategies in New Zealand. Two other natural factors are also important in New Zealand; topography and wind. There is plenty of evidence to show that trees on exposed, windy slopes or ridges can exacerbate spread problems by disseminating seed further than trees on ¯at sites. It is not only the strength but also the direction of wind that is important. Although the prevailing wind is from the west and therefore spread is usually to the east, there is evidence to suggest that spread elsewhere has eventuated after strong wind events from other quarters. Richardson (1998) suggests that the negative impacts of self-sown stands are similar in most respects to plantations. While impacts such as increased biomass, shifts in life-form and structural diversity, and changed nutrient cycling are common to both planted and self-sown pine stands, there are important differences between areas being invaded by wildings and those which have been planted. Plantations are usually `blanket' planted, with site preparation ensuring that trees are established wherever possible. Self-sown trees are much more site speci®c as to where they grow, with seedlings having dif®culty establishing within thick vegetation or excessively cold, dry or moist sites. The result is that self-sown areas can be more appealing visually (tree location more in sympathy with natural topography), biodiversity is richer, and natural hydrology is less disrupted. These factors can be important elements in the public's acceptance (or otherwise) of introduced conifers. It is possible that some of the spreading vigour of lodgepole pine in New Zealand could be explained by the development of a local landrace. This has been reported in other countries (Ledig, 1998). In seedling

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provenance trials, Ledgard (1980) found that lodgepole pine seedlings from local seed from parents of unselected origin, outperformed 12 other North American origins collected from 378 to 588 latitude in the species home range. The improved performance could result from natural selection for adaptation to New Zealand conditions, and/or from inter-provenance `hybridisation', as recognised seed stands are comprised of a mix of either selected superior individuals or best provenances. 9. Conclusion Many introduced conifers grow well in New Zealand, and they form by far the major component of the country's successful commercial forest industry. The majority regenerate naturally, with the most vigorous spreading species being lodgepole pine, probably due to its coning at a younger age, its ability to produce large numbers of seed and wildings (even above natural treeline), its lighter seed allowing dispersal over wide areas, and the low mortality of young seedlings once established. The relative predictability of spread patterns allows for ready calculation of spread risk and the development of long-term control plans. Control strategies focus on keeping trees clear of exposed (seed take-off) sites and spread susceptible (lightly grazed or vegetated) areas, managing surrounding land to minimise seedling establishment opportunities (particularly through grazing), and removing any outlier wildings before they produce seed. Where large areas of wildings exist, containment (as opposed to removal) is often the most practical control option. Concern about wilding spread is a major reason why lodgepole pine has rarely been planted in recent years, and experience with this species has highlighted the negative aspects of natural regeneration generally. The reaction of both the public and land administration agencies has been inconsistent, probably due to a lack of knowledge about the facts concerning conifer spread. A programme of education and awareness to promote spread risk avoidance and control by landowners themselves is needed. If successful, the risks of unwanted wilding spread from both new and existing plantations, woodlots and shelterbelts will be readily minimised.

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